1. Field of The Invention
This invention relates to polymer-based fluids for use in boring, excavating and trenching operations in the deep foundation construction industry, the subterranean construction industry, and in tunneling, in well drilling and in other applications of earth support fluids.
2. Description of Related Art
In creating foundations and shorings for buildings, bridges and other structures, and in excavating and subterranean boring for installation of utilities, barrier walls, transit ways and drainage systems, fluids have been used to support the surrounding earth during excavation operations. Whenever subterranean construction or excavation must be accomplished in granular, unstable, water-saturated or gas-charged earth formations, it has been customary to fill the boreholes, tunnel faces or excavations during the excavating or drilling process with water-based earth-support fluids, also known as slurries or muds. These fluids have consisted essentially of water, a thickening and/or filter-cake-building agent and earth solids from the excavation.
The conventional materials for making these fluids are clays such as bentonite and attapulgite. More recently, water-soluble polymers have been introduced and used in place of or in combination with such clays. (In this document "water-soluble", "soluble", "dissolved" and similar terms describing the "dissolution" of polymers refers to polymers that form either true solutions or colloidal dispersions. With regard to polymers, we use the concepts of water solubility and water dispersibility to colloidal dimensions interchangeably.) A widely used type of water-soluble polymer in these applications is a partially-hydrolyzed polyacrylamide (PHPA), in the form of a primarily linear, or non-crosslinked, long chain polymer with an anionic charge density of twenty to thirty mole weight. This type of polymer is available in liquid water-in-oil emulsion form and in dry form. Other polymers include guar gum, xanthan gum, cellulosic polymers, starch and blends of these. All of these polymers, including the PHPAs, have been prepared and applied in ways which are intended to accomplish complete dissolution or colloidal dispersion of the polymers when mixed with water to prepare earth support fluids.
With acrylic-polymer-based fluids containing no bentonite or other cake-building inorganic colloids, fluid loss control has been unattainable or poorly realized. The dissolved or colloidally dispersed water-soluble polymers, when selected and used according to past practice, could not plug the pores in the granular soil or create a filter cake as can bentonite and inorganic colloids. It has been impossible to adequately control fluid loss without adding mineral colloids or finely-divided materials such as native clays and silts incorporated into the slurry from the excavation.
In previous efforts to excavate when using acrylic-polymer-based systems, there has been little correlation or matching of the viscosity or polymer concentration of the fluid to the characteristics or stabilization requirements of the soil or earth formation being excavated, or to the requirements for concrete-to-soil bonding for frictional load transfer. Essentially the same viscosity range and polymer concentration have been used regardless of the nature of the formation being excavated. This indicates a lack of understanding of the properties and performance achievable with the use of polymers.
In the polymer slurries there has been no counterpart of the bentonite filter cake. That is to say, there has been no effective means or medium--equivalent to the bentonite filter cake--of transferring hydrostatic pressure to the surrounding soil, or of controlling the loss of the slurry into permeable soil. The polymer slurries have been used with limited success, providing performance intermediate between simple water and a bentonite slurry. The lack of a pressure differential (or pressure drop) across a relatively thin medium at the excavation/soil interface prevents the efficient transmission of hydrostatic pressure to the walls of the excavation. Instead of this pressure being brought to bear at the wall of the excavation, which results in a supported wall, the pressure is diffused throughout the pore system of the soil near the excavation, resulting in lack of directional support of the soil. High rates of fluid loss result in large volumes of the slurry saturating the soil radially or laterally around the excavation. This saturation of the soil by fluid that is in near-perfect communication with the fluid in the excavation creates a zone around the excavation of essentially equalized pressure.
This equalization of pressure, and the attendant failure of the slurry to exert directional force against the soil, can result in collapse of the excavation. In dry or non-water-saturated granular or permeable soils, the lubricating effect of saturating the soil around the excavation with invaded slurry also contributes to the likelihood of collapse. These problems of excessive permeation by low-viscosity polymer fluid are especially severe when emulsion polymers are used in preparing the fluid, due to the oil and water-wetting surfactants in the emulsions. Deep permeation of a dry sand by polymer fluid containing emulsified oil and water-wetting surfactants can aggravate de-stabilization of the soil by fully wetting the grain-to-grain contact points and lubricating the grains of soil, such that any slight natural cohesion is destroyed and the soil collapses. Thus the absence of a means of transferring hydrostatic pressure, and the related lack of control of fluid loss into the soil, are serious detriments to the slurry excavation process and represent a major weakness in polymer slurry technology as previously practiced.
When using polymer-based systems, control of fluid loss has been achieved by adding bentonite, silts, and/or other available fines or colloids to the fluid or by boring native silts and clays in an attempt to form a mineral-enhanced filter cake on the face of the excavation. When a mineral-based or mineral supplemented slurry is used in fine-grain sands, the dispersed mineral colloids in the slurry can provide improved control of fluid loss because the pores in the soil are small. But mineral-based and mineral supplemented slurries, due to the thick filter cakes they create, reduce borehole or excavation gauge. This reduced gauge can reduce the diameter or thickness of formed or cast structures created in the excavations and boreholes. Similarly, mineral-based and mineral supplemented filter cakes can negatively affect the geometry of the formed or cast structures. Additionally, mineral-based or mineral supplemented filter cakes, as a sheath of continually reactive and hydratable colloids at the interface between the concrete and surrounding earth, can reduce skin friction, or perimeter load shear, on which formed or poured structures rely for their load-bearing capacities. Reduced friction may promote instability, movement and stress on these structures, which can damage the subterranean structure and the super-structure that rests on them.
Historically, polymers were designed and intended to mix without forming masses or pearls of undissolved, incompletely dispersed or semi-hydrated polymer. It has been customary to prehydrate and maximize solubilization, homogenization and colloidal dispersion of these materials before introduction of the fluid into the excavation or borehole. This has been accomplished through the use of induction systems, recirculation, agitation, and processing of the polymer, and retaining the prepared fluid in a maturation tank for a period of time prior to introduction of the fluid into the excavation or borehole.
Clay slurries or muds are formulated with about five to ten percent bentonite in fresh water or about five to ten percent attapulgite in salty water. Whether the slurries are formulated with clays or polymers, the object is to create a viscous and/or dense (high specific weight) fluid that stabilizes and supports the walls of the excavation, excludes groundwater and gases from the excavation, and facilitates the progress of the construction project. A key to success in these efforts is to avoid loss or seepage of the excavating fluid into the surrounding earth during the excavating operation. If the fluid is lost into the earth formation and the excavation cannot be kept full of fluid, the excavation can collapse and groundwater or gases can enter the excavation. Excessive fluid loss can also disrupt naturally occurring cohesive forces between the formation solids.
The viscosity of the polymer-based earth support slurries has been maintained by design generally in a range of about 30 to about 45 seconds per quart as measured with a Marsh Funnel according to viscosity measurement procedures standardized by the American Petroleum Institute. This range of viscosities was felt, in light of "industry knowledge" of the prior art, to be the most effective and least damaging. Builders and engineers have expressed concerns that polymer slurries, especially high-viscosity polymer slurries, might interfere with the bonding of concrete to soil, thus degrading the frictional load-bearing capacity of structures such as bored piles. They have also expressed concerns that higher concentrations of polymer might interfere with concrete-to-rebar bonding and concrete compressive strength development.
These concerns, along with the perceived need to keep the cost of polymer fluids comparable on a unit volume basis with the cost of bentonite slurries, and the lack of appreciation of the fluid loss control and soil-stabilizing performance obtainable with high-viscosity polymer fluids, has caused polymer fluid viscosity and dosage recommendations to be held in low ranges.
The cohesion of granular earth solids exposed in-situ by slurry-excavation or loaded onto or into excavating tools is governed, aside from the inherent cohesion of the soil, by the earth binding capacity of the slurry and maintenance of pore water pressure and intergranular stresses. The earth binding capacity is the composition's affinity for earth solids, which causes the earth support fluid to chemically and physically bond or attach to exposed and excavated earth, both on the excavation tool and on and within the excavation walls. This property also preserves or improves the tendency of grains of earth to hold together in mass as opposed to separating into individual grains or smaller masses (improved cohesion). This improved cohesion aids in loading excavation tools and conveying of the earth solids up from the excavation. When an earth support fluid of high earth binding capacity permeates granular or permeable soil, it can preserve or improve the cohesion of the soil, which helps stabilize the excavation. The earth binding capacity is also manifested as the ability of the earth support fluid to film or encapsulate clay-bearing mineral solids and thereby reduce their tendency to adsorb, absorb or take up water.
When the conventional liquid emulsion PHPA is used as the primary slurry additive, it has occasionally been added directly into the borehole or excavation, and the drilling or excavating tools have been used to mix it with water and/or fluid in the borehole. However, this is not the industry-preferred method for emulsion PHPA addition, because it can produce agglomerates of undispersed, wasted polymer. These agglomerates have been documented to cause voids in concrete due to becoming intermingled with the concrete during placement and then slowly degrading, leaving holes or honeycomb structures within the concrete. Typically and preferably an reduction unit or in-line mixer, sometimes in conjunction with a hydration tank with recirculating pumps is utilized to assure complete solubilization or dispersion of the polymer prior to introduction to the excavation or borehole.
The ability to rapidly mix and yield polymer directly in the borehole or excavation is advantageous because it eliminates the need for costly, cumbersome mixing and processing equipment. It can significantly reduce time required to drill, excavate and construct piers, walls, pads, wells, etc.
Whenever polymer has been used, a primary objective in mixing polymer into water or earth excavation fluid has been to create a homogeneous solution or mixture and to accomplish complete dispersion and dissolution of the polymer as readily as possible. Completely dissolving and homogenizing the polymer in the water or fluid has been considered a key to optimum performance. Incompletely homogenized polymer of any kind, whether in the form of agglomerates, polymer strings, "fisheyes," gels, microgels, pearls or masses has been seen as disadvantageous and wasteful. Avoiding the presence of incompletely hydrated polymer in the slurry has been a prime objective of fluid design and mixing practice.
At the relatively low polymer doses and/or viscosities previously used in acrylic polymer slurries, polymer depletion can occur when silty or clay-bearing soils are excavated. Earth solids disturbed by the excavating tool are mechanically dispersed into the slurry. The increased solids surface area thus exposed creates an increased demand for polymer as the reactive sites on the solid particles attract and bond to the oppositely charged sites on the polymer. This ionic bonding of polymer to dispersed solids depletes the concentration of dissolved or colloidal polymer in the system, such that the properties of the system become governed increasingly by earth solids content and less by the solution properties of the polymer. In a polymer-depleted, solids-laden system Marsh Funnel viscosity values can be maintained in reasonable ranges by the hydration and dispersion of native days, which in a high-concentration polymer system would not be allowed to hydrate and disperse. This solids-based viscosity can give a false sense of security because the viscosity of the system is in expected ranges but there is insufficient polymer available to stabilize the excavation.
The depletion of the polymer in the system allows further hydration of the walls of the excavation, with attendant de-stabilization. At the low remaining concentration of polymer, the polymer acts as a flocculant instead of a viscosifier and stabilizer/protective coating agent. In the flocculated system the flocs of fine solids settle toward the bottom of the excavation and can form a soft mass of non-cohesive material which is difficult to remove with excavating tools. This soft material, if not removed, can create voids in formed concrete and can be detrimental to the integrity and bearing capacity of cast-in-place concrete structures formed in the excavations.